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DURHAM, N.C. -- The more children a person has, the greater the risk he or she will become obese, according to a new study from Duke University Medical Center. From an analysis of a large database of middle-aged Americans, researchers found women faced an average 7 percent increased risk of obesity per child and men an average 4 percent increased risk per child. Researchers attribute the weight gain to a busier lifestyle that may include a diet of more fast food and leave less time for exercise.

"As families grow, parents need to be educated about the importance of exercise and a healthy diet," said Lori Bastian, M.D., associate professor of medicine at Duke University Medical Center and research associate at the Durham VA Medical Center, co-author on the paper. "Obesity is a family problem because children follow the lead of their parents. A healthy lifestyle for one is a healthy lifestyle for all."

The research appears in the January/February 2004 issue of the Journal of Women's Health.

According to the National Health and Nutrition Examination Survey, 30 percent of all Americans are obese. Obesity is linked to several major health concerns including diabetes, heart disease and some cancers. An estimated 300,000 adults die of obesity-related illnesses every year in the United States.

Women often cite having children as a cause of weight gain, and may attribute the gain to physiological changes associated with pregnancy. Several previous studies have indicated a possible relationship between obesity and the number of children a woman has. However, the Duke study is the first to examine the association between parenthood and obesity in both mothers and fathers.

"Increased risk of obesity in both men and women suggests a substantial portion of the effect of obesity related to parenthood has to be social, cultural or psychological," said Bastian. "It's difficult to imagine a physiological mechanism through which men could gain weight during pregnancy or after childbirth. Further studies are needed to isolate cause and effect so we can more accurately suggest target groups for obesity prevention and research."

The researchers examined data from the Health and Retirement Study (HRS), a national survey designed to study health, social and financial issues in middle-aged Americans. The HRS study, which collected data from 1993 to 2000, includes information on more than 12,600 Americans. Most respondents were aged between 51 and 60 in 1992. The study is a cooperative agreement between the Institute for Social Research at the University of Michigan and the National Institute on Aging.

For the Duke analysis, the researchers used data only from the baseline survey completed in 1992. Only married respondents and spouses between the ages of 40 and 70 with children were included in the analysis, which included a total of 9,046 men and women (4,523 couples). Single or divorced individuals with children were excluded in the analysis. The number of children reported by the couples ranged from one to 19, and included both biological and adopted children. Of the sample, 79 percent were white, 12.5 percent were African American and 8.5 percent were Hispanic.

The researchers acknowledged possible limitations in the study. The data collected were based on self-reported weight and height, although the researchers said other studies have shown such data to be accurate. Also, the data did not include a parent's body size prior to childbearing or the biological relationship of the children to the couple.

Because the researchers focused their analysis exclusively on the connection between number of children and obesity, they controlled for age, household income, race and ethnicity, work status, physical activity, tobacco use and alcohol use.

"After adjusting for all these factors, the number of children played a statistically significant role in the obesity of both men and women," said Truls Ostbye, M.D., Ph.D., senior author on the paper and professor of community and family medicine at Duke University Medical Center.

Ostbye said while the increased risk for obesity in women was 7 percent for each additional child and only 4 percent for men, the difference between these two figures was not statistically significant.

"Obesity associated with number of children is not just a problem linked to physiological changes in women during pregnancy," said Ostbye. "There are social, cultural or psychological mechanisms that bring about this weight gain, and this is illustrated by our results that showed men were also at a greater risk of obesity."

The researchers say their findings point to a significant public health issue that must be addressed.

"Having children can be a wonderful experience for couples, but parents need to be aware of their increased risk of obesity and receive appropriate counseling as their families grow," Bastian said. "It's important to the overall health of the family that diet and physical activity take a significant role in the family's lifestyle."

Bastian says additional research is needed to examine the changes in attitudes toward diet, physical activity and exercise that occur among couples caring for small children. She said researchers and clinicians need to learn the causes of changes in diet and physical activity that may increase the risk of obesity, and to develop appropriate interventions.

First author, Haoling Weng, M.D., is an internal medicine resident at Duke University Medical Center and is affiliated with Durham VA Medical Center. Co-authors include Donald Taylor, Ph.D., Center for Health Policy, Law and Management in the Terry Sanford Institute of Public Policy at Duke University, and Barry K. Moser, Ph.D., Durham VA Medical Center and Duke Comprehensive Cancer Center.

The research was supported by grants from the National Institutes on Aging and a VA Career Development Award from Health Services Research and Development.